On Some Meteorological Patterns in the Dead Sea Area

during Advective Sharav Situations

Mordecay Segal,1 Ytzhaq Mahrer2 * and Roger A. Pielke I

I Department of A tmospheric Science, Colorado State University, Fort Collins, CO 80523 USA; and2 Cooperative Institute for Research in the Atmosphere (CI RA), Colorado State University,

Fort Collins, CO 80523 USA

AbstractWind, temperature and moisture fields, as well asevaporation rates for the advective sharav condition,were simulated along the Dead Sea region. Thepossible effect of the stable thermal atmosphericstratification on evaporation patterns over the DeadSea is discussed. This contribution concludes a seriesof mesoscale modelling studies over Israel. Furtherrefinements of the current model to be consideredsubsequently are outlined.

IntroductionThe proposed construction of a ~hannel carryingwater from the Mediterranean to the Dead Sea, forgeneration of electric power (Weiner, 1980), moti-vated the model analysis presented here. Evaluationof evaporation rates from the lake, while consideringthe temporal and spatial character of the pertinentmeteorological fields, is a crucial component of sucha project. Such an evaluation can be achieved bydetermining the contributions of the most frequentclimatic situations in the Dead Sea area to evapora-tion. Previous studies evaluating the meteorologicalpatterns in this area focused on the two most signi-ficant seasons, summer and winter (observationalstudies by Ashbel (1939) and Bitan (1974, 1977) andthe numerical model study by Segal et al. (1983)).The current contrib~tion evaluates a less prevalentsituation -the advective sharav. This synopticsituation, which is associated with a dry and warmeasterly flow over the region, is frequent during thefall and spring (for a general description see Levi(1967) and Winstanley (1972)). A schematic illustra-tion of surface pressure systems during such con-ditions is given in Figure 1. During these conditions,a distortion of the regular daily cycle of thermallyinduced circulations is typical. The onset of a veryintense, stable marine layer when a warm flow movesover a cooler water body was indicated in a previous

*Perrnanent affiliation: The Seagram Centre for Soil andWater Sciences, The Hebrew University of Jerusalem, Facultyof Agriculture, P.D.B. 12, Rehovot 76100 IsraelReceived May 22, 1983 and in revised form November 23,1983

DEAD SEA ADVECTIVE SHARA V 77

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approximation for the model may cause some intensi-fication of the simulated flow in the vicinity of thelake area as compared with reality (Alpert et al.,1982; Segal et al., 1983). However, it has the ad-vantage of enabling the use of fine grid resolutionover the lake area, which becomes expensive whenapplied in a 3-D model. Verification studies for thismodel over irregular terrain (e.g. Mahrer & Segal,1979b; Mizzi, 1982; Segal et al., 1982), however,indicate reasonable accuracy of prediction even withthose assumptions.

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(a) cb>Fig. 1. Schematic illustration of two typical surface pressuresystems associated with an advective sha'rav situation (basedon Levi, 1967).

II

ResultsThe model simulation commenced at 2000 LSTfollowing a dynamic initialization of 6 h. The initialconditions (Fig. 2) represent typical advective sharavconditions in the sense stated in the previous section.They were based on vertical radiosonde profiles atthe Bet Dagan radiosonde stations, as well as oninformation given in the papers of Levi (1967) andWinstanley (1972). The Dead Sea water surfacetemperature, Ts, is 26.7°C which is typical for theMay-averaged temperature (Ashbel, 1964). Thesimulation was carried out using a horizontally nestedgrid of 2.5 kIn over the central part of the domain.The model-simulated cross-section is illustrated inFigure 3 (the cross-section passes through Ashdod to

"

similar synoptic situation relating to the Mediter-ranean coastal area (Mahrer & Segal, 1979a). Helice,it is suggested that stable surface layers are alsoexpected over the Dead Sea, thereby effecting theevaporation rate from the lake.

We present here model simulated flow, tem-perature and moisture patterns over the Dead Sea,while emphasizing the simulated rate of evaporation.The model formulation (including its boundaryconditions) is given in detail in Pielke (1974) andMahrer and Pielke (1977, 1978), and further informa-tion relating to the model application over the DeadSea is found in Segal et al. (1983).

The synoptic conditions typical of an advectivesharav are not necessarily steady when they prevailfor more than 24 h. The flow direction and intensity,and temperature and moisture pattern generally varyfrom one case to another. However, there are sub-stantial similarities between episodes. Hence, in thecurrent study, where steady synoptic conditions areassumed, orily a preliminary insight is suggested.

It should also be stated that from a modellingpoint of view, because of the steepness of the terrain,the use of the hydrostatic equation may not becompletely consistent. Hence, the representation ofthe pressure distribution in the current model is likelyto introduce some inaccuracies in the simulationresults (pielke & Martin, 1981; Wipperman, 1981),although further studies with a more complete model(i.e. norihydrostatic and the complete transformedequations) are needed to document quantitatively theinfluence of the assumptions. Also, the use of the 2-D

() i 2 :3 "4 5 67 8 g-m/S

Fig. 2. The initial vertical profiles of wind direction, wind

speed, temperature and specific humidity.

78 M. SEGAL ET AL.

§C5+-<>..,-'w

Fig. 3. Schematic illustration of the terrain features used for the model simuJation. The dar.ker line indicatesthe fine grid resolution section.

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Flow PatternsFigure 4 presents several selected flow patterns.

An indication of the large scale vertical profile offlow can be obtained from the vertical profile at theeastern boundary. A return flow or a major {eductionof the wind speed aloft along the eastern slopes is

the west and Mizpe Shalem to the east; the heavy lineindicates the nested grid section).

Simulated results near midnight of the secondnight are also presented, largely to indicate theexpected differences from the first night due todifferences in the anteceding daytime conditions.

DEAD SEA ADVECTIVE SHARAV 79

evident throughout the simulated period, attributedmostly to the creation of a pressure gradient which iscounter to the large-scale flow direction in the leeof the slopes. As expected, towards the end of thenight, 0500 LST, the strongest nocturnal surface flowis along the eastern slopes, due to the superpositionof a thermally induced drainage flow with a dynami-cally intensified large scale flow. Along the westernslopes, the interaction of the nocturnal drainage flowand the large scale winds mostly effects the directionof the resultant flow. As the slopes become heated(0800 LST), and the thermal stratification therebecomes unstable (as will be shown later), a reductionin the surface layer wind speeds as compared to thenocturnal flow is predicted along both slopes. At1400 LST the effect of the thermally induced flowby the lake and the mountains is pronounced overthe domain. Most pronounced is the interaction ofthe mesoscale induced flow with the large scale flowalong the upper eastern slopes. The onset of typical

nocturnal drainage flows in the surface layer is notice-able towards the second night, as illustrated for 2300LST. It is worth noting that while evaluating thee'fitire simulation period, substantial changes in theflow occurred mainly below the 1000 m height, withthe general pattern of the flow almost unchangedabove this elevation throughout the entire simulation.

Ashbel (1939) states that during the advectivesharav situation: "Over the whole of the Dead Seathere is a north wind, but this generally gives placeabout noon to sea breeze." Such behavior for the sur-face flow is also indicated by our simulation results.

Temperature PatternFigure 5 presents the predicted temperatures at

the given vertical cross-section. The creation of athermally stable surface layer along the mountainslopes due to radiational nocturnal cooling is wellpronounced towards the end of the night (0500LST). Due to the very dry atmosphere, the relatively

M. SEGAL ET AL80

TABLE 1. Evaporation Rates from a Water Body in Units of cm/24 h for Various Combinationsof AI). Aq and Wind Speeds (VI. as Computed by the Procedure Described in the Text

(Water Surface Temperature 26.TC)

~q

(g/kg)Um/s

~O(OCI -10 -12 -14 -16 -18 -20 -22 -24-8-2 -4 -6

.01

.02 .03 .04 .06 .07 .08 .09.10 .11 .12 .13

.00 .00 .00 .00 .01 .01 .01 .01 .01 .01 .01 .01

.00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00

.00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00

.00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00 .00

.07 .14 .21 .28 .35 .41 .48 .55 .62 .69 .76 .83

.05 .10 .15 .19 .24 .29 .34 .39 .44 .49 .53 .58

.03 .07 .10 .1 3 .17 .20 .23 .27 .30 .33 .37 .40

.02 .04 .07 .09 .11 .13 .16 .18 .20 .22 .25 .27

.02 .03 .05 .06 .08 .09 .11 .12 .14 .15 .17 .18

.15 .30 .44 .59 .74 .89 1.03 1.18 1.33 1.48 1.62 1.77

.13.25 .38 .51 .64 .76 .89 1.02 1.15 1.27 1.40 1.53

.11 .22 .33 .44 .55 .66 .76 .87 .98 1.09 1.20 1.31

.09.19 .28 .37 .47 .56 .65 .75 .84 .931.031.12

.08 .16 .24 .32 .40 .47 .55 .63 .71 .79 .87 .95

.24 .48 .71 .95 1.19 1.43 1.67 1.90 2.14 1.38 2.62 2.86

.22 .44 .66 .88 1.10 1 .32 1.54 1.76 1.97 2.19 2.41 2.63

.20 .40 .61 .81 1.01 1.21 1.41 1.62 1.82 2.02 2.22 2.42

.19.37 .56 .74 .931.11 1.301.491.671.862.04 2.23

.17.34 .51 .68 .85 1.02 1.19 1.36 1.53 1.70 1.87 2.04

.34 .68 1.02 1.36 1.70 2.03 2.37 2.71 3.05 3..39 3.73 4.07

.32 .64 .97 1.29 1.61 1.93 2.26 2.58 2.90 3.22 3.55 3.87

.31 .61 .92 1.22 1.53 1.84 2.14 2.45 2.75 3.06 3.37 3.67

.29 .58 .87 1.16 1.45 1.74 2.03 2.33 2.62 2.91 3.20 3.49

.28 .55 .83 1.10 1.38 1.65 1.932.21 2.48 2.76 3.03 3.31.

222224444466666

2468

102468

102468

102468

102468

10

810

10101010

where: Pa = air density, u * = friction velocity andq * = flux specific humidity. Based on the Businger et

al. (1971) formulation for the surface layer and theroughness parameter evaluation over water surfacesas adopted by Mahrer and Pielke (1977) we estab-lished a table of E values. These values of E asdependent on the wind speed at 10 m height and thedifferences in potential temperature (AfJ) and mois-ture (Aq),

strong solar radiation causes an unstable boundarylayer of several hundred meters depth at theselocations by 0800 LST. Sharp stabilization of thelower layer over the lake, as warm air descends overthe relatively cool lake water, occurs during thedaylight hours (1400 LST). Because of the previousvery warm day, the cooling effect during the firsthalf of the second night is insufficient to reducetemperatures to the previous night's temperaturelevel. Consequently, a very stable atmosphere overthe lake is established (2300 LST).

l18 = 8(z =10 m)- 8(zo)

l1q = q(z = 10 m)- q(zo) ,(2)(3)

where Zo is the water surface roughness parameter,are given in Table 1. Over the water, Zo is propor-tional to u; and its magnitude is usually less thanseveral mm. The values given in Table 1 refer to 24-hrates of evaporation for various combinations ofthermally stable stratifications and deficiencies inmoisture over the water. The water surface tempera-ture is assumed to be 26.7°C in these calculations.In real situations, some of the extreme combinations

Moisture FieldEvaporation rates from water surface. Prior to

examining the predicted moisture pattern, it isdesirable to evaluate the patterns of evaporation

,Afrom a water body as a function of meteorologicalconditions.

The rate of evaporation (E) from a water surfacecan be approximated as:

E = Pau.q. (1)

DEAD SEA ADVECTIVE SHARA V 81

~. BOO LS1SPECIFIC HUMIDItY

20002000

'-"--15001500

1000

gN

500

1000

~...

500

/\

-5:0500

might not be feasible. In addition, evaporation rateswould generally change with time because of modi-fications in surface layer properties. However, in ourcase the computed values can provide some scalingassessment of the evaporation character in a marineatmosphere. In our simulation the most importantphenomena are: (1) reduction in evaporation asthermal stability increases; (2) increase in evaporationrate with intensification of the flow (due to theincrease in the u. values); (3) increase of thermalstability is relatively more effective in reducingevaporation rates with light flows than with strongerwinds, and (4) as the wind speed increases, the effectof small changes in ~ has a more pronouncedeffect on the evaporation rates as compared to asimilar percentage change in dO.

over the lake (its value tends to reduce sharply asthe thermal stratification becomes stable); (3) mois-ture content in the air at the lake environment;(4) horizontal advection of moisture offshore.

It is evident that as the simulation proceeds andthe vertical eddy diffusion reduces, the accumulationof moisture within the surface layer is somewhatenhanced (Fig. 6). The flow patterns indicate adominant northerly flow over the lake. However,the infinite lake length imposed by the 2-D require-ment makes impossible the removal of moisture fromthe lake by a longitudinal advection, while in reality,some moisture removal should be involved. Hence,some overestimation of the moisture field over thelake will result. A minor increase in moisture as thesimulation progresses is calculated for the upperlayers. This moisture increase is apparently due tovertical diffusion and advection from the lake surface.(The existence of a convergence zone over the lakeis not shown; however, it is implied from the patternof horizontal winds.)

Model predicted moisture patterns. The moisturedistribution in the atmosphere over the lake dependson several factors: (1) rate of evaporation from thelake; (2) vertical eddy diffusion in the atmosphere

82 M. SEGAL ET AL.

TABLE 2. Evaporation Rates along the Dead Sea Cross-Section (cm/24 h) at the Water PointsBased on 24 h Model Computation Beginning at 2300 LST; See Fig. 2 for Points Locetion)

Grid point3 4 5 6 Average2

Advective sharav (es/es = 0.68)0

Summer day (es/eso = 0.9)

Summer day (es/eso = 0.68)

0.510.880.47

0.500.760.39

0.430.650.32

0.390.620.33

0.390.520.31

0.430.460.25

0.440.650.35

Daily simulated evaporation rates. Table 2 givesthe daily rates of evaporation from the lake at the sixwater grid poipts. In the evaporation rate calculationit has been assumed that the saturated water vaporpressure

for the Dead Sea is given according to

es = ()~R (4)

negligible increase in the evaporation rates. Hence,while the calculated evaporation rates are regardedas qualitatively representing the cross-lake spatialdistribution, their absolute values should be regardedas underestimates.

Summary and ConclusionsA model simulation of advective sharav flow, tem-perature, moisture and evaporation patterns hassuggested a preliminary evaluation of these charac-teristics 'for the Dead Sea area. The effect of thedynamic and thermal forcings on the meteorologicalfields over the Dead Sea region was discussed. Twosupportive factors, the relatively dry environment andthe intense wind speed, tended to counteract theeffect of the highly stable lower atmosphere duringthe major part of the simulation, which resulted ina moderate daily rate of evaporation from the lake.

As -itated in this paper, a quantitative evaluation ofthe daily evaporation rates should consider, foradvective sharav cases, a 3-D model simulation,including the modelling of the thermal characteristicsof the upper lake layer. In the current simulation thecomputed evaporation rates are likely to be under-estimated, although their cross-lake distrib~tion isregarded as being adequate. While the evaporationrates appear to be sensitive to the water temperatureand overlying thermal structure and moisturecontent, such a sensitivity is not, however, expectedin the flow and temperature fields when warm airflows ov~r cold water as implied by the studies ofMizzi (1982) and Segal and Pielke (MS).

In view of the lack of detailed observations, thepresent study provides a preliminary evaluation ofthe local Dead Sea meteorology during advectivesharav conditions. It may also be helpful in theplanning of an observational program.

e ,So

where eso is. the saturated vapor pressure over a purewater surface, and es corresponds to that of the DeadSea surface with its high salinity. Segal et al. (1983),however, computed the evaporation rate over theDead Sea for a summer day while using the high ratioof es/eso = 0.9. For purposes of comparison, we

resimulated the summer case, while utilizing (4) forthe computations of the evaporation rates. Thecomparison implies somewhat higher values for the24 h amount of evaporation rate in the advectivesharav case. It is worth noting the high sensitivityof the evaporation rates to the value of es/eso asobtained for summer cases.

Onshore removal of moisture originating over theDead Sea is possible only along the model cross-section direction (east-west). As stated earlier,removal of moisture in the longitudinal directionis not considered because of the 2-D assumptioninvolved with the current study. Scaling the moistureadvection in both directions suggests, however, thatthe east-west advection dominates the longitudinalone if UY/VX ~ 1 (where U -scaled surfa~e east-west wind component; V -scaled surface north-south wind component; X -scaled sea width; Y -

scaled lake length). Since Y/X ~ 5 for the Dead Sea,then U should be about twice V if the longitudinaladvection is to be of reduced importance. Thiscondition is only partially fulfilled through the courseof the simulation. Hence, the model predictions forLlq should be considered somewhat higher than thosewhich would occur in the real atmosphere.

Examining Table 1 for typical simulated valuesof Llq (= 7 gfkg) and typical sea surface wind speedsimplies that a small increase of Llq can cause a non-

AcknowledgementsWe acknowledge the Atmospheric Sciences Section ofthe National Science Foundation for grant #ATM-8242931. Computer simulations presented in this

83DEAD SEA AD VECTI VE SHARA V

study were performed at the National Center forAtmospheric Research, which is supported by theNational Science Foundation. We would like to thankD. Anati and M. Weiss for their comments. Theauthors also wish to thank Sara Rumley, who pro-vided her usual outstanding editorial and secretarial

support.

by mountains -the Dead Sea case. Q. J. R. Meteorol.Soc. 109: 549-564.

Segal, M. and R.A. Pielke. On the effect of water temperatureand synoptic wind speed on the development of lakebreezes (to be submitted to J. Geophys. Res.).

Weiner, D. 1980. The Mediterranean-Dead Sea project: amathematical model and dynamic optimization of solarhydroelectri~ power plant. J. Solar Energy Eng. 102:281-286.

Winstanley, D. 1972. Sharav. Weather 27: 146-169.Wipperman, F. 1981. The applicability of several approx-

imations in meso-scale modelling -a linear approach.Contrib. Atmos. Phys. 54: 298-308.

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